It is known that dislocation density of semiconductor compounds of columns III-V and II-VI of the periodic chart can be reduced by proper introduction of alloying atoms into the compounds. In Sher U.S. Pat. No. 4,697,202 and Mooney et al U.S. Pat. No. 4,568,792 it is indicated that the dislocation density of III-V and II-VI semiconductor compounds can be reduced by alloying such semiconductors with isoelectronic impurity atoms forming bond lengths with the semiconductor atoms that are less than the bond lengths between the atoms of the semiconductor. It is also known to add indium as an isoelectronic impurity for gallium arsenide dislocation density reduction. The indium is added to a melt during gallium arsenide crystal growth. The indium-arsenide bond is longer than that of the gallium arsenide whereby indium alloying of gallium arsenide is apparently by a mechanism different from that relied on in the aforementioned patents. The present invention is applicable to both long and short bond length alloying atoms.
The conventional method of growing semiconductors having dislocation density reducing alloying atoms in the semiconductor structure is by melt techniques, such as modified Bridgeman and Czochralski techniques. These prior art techniques generally limit the number of the alloying atoms to relatively low values such that the solubility limit of the host semiconductor compound cannot be exceeded. This value is insufficient to eliminate dislocations totally. For example, these conventional melt techniques are generally able to introduce a maximum of about 0.09 atom percent boron into gallium arsenide, to form Ga.sub.0.9991 B.sub.0.0009 As. In the conventional methods, for boron concentrations in GaAs above 0.09 atom percent solubility limit, constitutional super cooling leads to striations and eventually polycrystallinity. It was found that in the region between striations dislocation density was relatively low. While the 0.09 atomic percent introduction of boron results in a significant dislocation density reduction, greater dislocation density reduction can be achieved with larger percentages of boron in gallium arsenide. The same is true for other alloying atoms that reduce semiconductor compound dislocation density. It would be desirable to introduce a number of alloying atoms such that the semiconductor compound is in a metastable state, exceeding the solubility limit thereof, and the alloying atoms are in a super-saturated state in the semiconductor compound. For example, it would be desirable to alloy gallium arsenide with boron so that there is a 5 atomic percent introduction of boron into the gallium, to produce Ga.sub.0.95 B.sub.0.05 As.
Reduction and elimination of dislocations in gallium arsenide have been sought for many years. If the high dislocation density inherent in gallium arsenide could be materially reduced over large volumes, the known advantages of gallium arsenide relating to high speed operation and radiation withstanding properties could finally be achieved. Hence, it is highly desirable to provide a virtually dislocation-free gallium arsenide carrier for active semiconductor devices, a result which could be achieved by introducing an epitaxial layer of gallium arsenide super-saturated with boron or nitrogen on a conventional substrate, e.g., of bulk gallium arsenide, silicon or sapphire. If the boron or nitrogen concentration in the gallium arsenide is supersaturated, the energy per unit length for dislocation formation arising mostly from long range shear strains increases in the presence of the short bond length induced increase of the elastic constants of the introduced boron or nitrogen atoms, to prevent dislocations from propagating from the substrate into the epitaxial layer. A problem, however, in introducing boron or nitrogen to a super-saturated concentration, is that the alloying atoms may affect the electric characteristics of the semiconductor devices formed in the epitaxial layer containing the alloying atoms. To obviate this problem, it is desirable to form a GaAs epitaxial layer on the epitaxial layer containing the alloying atoms. Also, great care must be exercised in depositing layers with supersaturated concentrations of the alloying atoms because of the great tendency for the alloying atoms to precipitate from the GaAs lattice structure into a conglomerated mass (inclusions).
It is, accordingly, an object of the present invention to provide a new and improved method of forming low dislocation density semiconductor devices and to the devices produced by such methods.
Another object of the invention is to provide a new and improved method of forming low dislocation density semiconductor devices wherein the device is alloyed with atoms that reduce dislocations in the semiconductor device.
Another object of the invention is to provide a new and improved method of forming low dislocation density devices wherein dislocation removing alloying atoms are above the solubility limit of a semiconductor compound.
Another object of the invention is to provide a new and improved method of alloying boron or nitrogen into gallium arsenide to an extent that the boron or nitrogen exceeds the solubility limit of the gallium arsenide and is supersaturated therein.